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This paper investigates reliable memberships and fundamental astrophysical properties of the old (∼8 Gyr) open cluster NGC 6791 using the astrometric and photometric data of the Gaia data release 2 (Gaia -DR2). We identify 2536 likely cluster members (≥0.6) in a five-dimensional (5D) parameter space using a Gaussian mixture model (GMM) clustering method. Based on these reliable 5D cluster members, we determine the core and tidal radii of the cluster to be R c = 3.69 ′ ± 0.23 ′ and R t = 16.96 ′ ± 2.29 ′ , respectively. We find evidence for the presence of mass segregation in the cluster. Among the likely cluster members, we identify six blue subdwarfs, 43 blue stragglers, and 27 red clump giants. We find that 34 blue stragglers (∼79%) lie within the core radius (< 3.69 ′ ) of the cluster, indicating that these stars may be formed by star collisions or by the evolution of primordial binaries within the dense core. The 27 red clump giants are used to accurately estimate the distance of the cluster. We determine a distance of 4118 ± 23 pc for the cluster using a Monte-Carlo simulation method. We confirm the presence of a significant zero-point offset (∼ − 0.055 ± 0.004 mas) in the Gaia parallaxes of the cluster members. Using the same simulation method, the mean proper motion and radial velocity of the cluster are found to be (〈 μ α cos δ 〉 , 〈 μ δ 〉) = (− 0.434 ± 0.008 , − 2.266 ± 0.010) mas/yr and 〈 V r 〉 = − 46.7 ± 0.6 km/s, respectively. [ABSTRACT FROM AUTHOR]

Context. Gaia Data Release 1 allows the recalibration of standard candles such as the red clump stars. To use those stars, they first need to be accurately characterised. In particular, colours are needed to derive interstellar extinction. As no filter is available for the first Gaia data release and to avoid the atmosphere model mismatch, an empirical calibration is unavoidable. Aims. The purpose of this work is to provide the first complete and robust photometric empirical calibration of the Gaia red clump stars of the solar neighbourhood through colour–colour, effective temperature–colour, and absolute magnitude–colour relations from the Gaia, Johnson, 2MASS, Hipparcos, Tycho-2, APASS-SLOAN, and WISE photometric systems, and the APOGEE DR13 spectroscopic temperatures. Methods. We used a 3D extinction map to select low reddening red giants. To calibrate the colour–colour and the effective temperature–colour relations, we developed a MCMC method that accounts for all variable uncertainties and selects the best model for each photometric relation. We estimated the red clump absolute magnitude through the mode of a kernel-based distribution function. Results. We provide 20 colour versus G − Ks relations and the first Teff versus G − Ks calibration. We obtained the red clump absolute magnitudes for 15 photometric bands with, in particular, MKs = (−1.606 ± 0.009) and MG = (0.495 ± 0.009) + (1.121 ± 0.128) (G − Ks − 2.1). We present a dereddened Gaia-TGAS HR diagram and use the calibrations to compare its red clump and its red giant branch bump with Padova isochrones. [ABSTRACT FROM AUTHOR]

Low-mass stars in their core-helium-burning stage define the sharpest feature present in the color-magnitude diagrams of nearby galaxy systems: the red clump (RC). This feature has given rise to a series of methods aimed at measuring the distributions of stellar distances and extinctions, especially in the Magellanic Clouds and Milky Way Bulge. Because the RC is easily recognizable within the data of large spectroscopic and asteroseismic surveys, it is a useful probe of stellar densities, kinematics, and chemical abundances across the Milky Way disk; it can be applied up to larger distances than that allowed by dwarfs; and it has better accuracy than is possible with other kinds of giants. Here, we discuss the reasons for the RC narrowness in several sets of observational data, its fine structure, and the presence of systematic changes in the RC properties as regards age, metallicity, and the observed passband. These factors set the limits on the validity and accuracy of several RC methods defined in the literature. [ABSTRACT FROM AUTHOR]

Context. Solar-like oscillations in red giants have been investigated with the space-borne missions CoRoT and Kepler, while pulsations in more evolved M giants have been studied with ground-based microlensing surveys. After 3.1 years of observation with Kepler, it is now possible to link these different observations of semi-regular variables. Aims. We aim to identify period-luminosity sequences in evolved red giants identified as semi-regular variables and to interpret them in terms of solar-like oscillations. Then, we investigate the consequences of the comparison of ground-based and space-borne observations. Methods. We first measured global oscillation parameters of evolved red giants observed with Kepler with the envelope autocorrelation function method. We then used an extended form of the universal red giant oscillation pattern, extrapolated to very low frequency, to fully identify their oscillations. The comparison with ground-based results was then used to express the period-luminosity relation as a relation between the large frequency separation and the stellar luminosity. Results. From the link between red giant oscillations observed by Kepler and period-luminosity sequences, we have identified these relations in evolved red giants as radial and non-radial solar-like oscillations. We were able to expand scaling relations at very low frequency (periods as long as 100 days and large frequency separation less than 0.05 μHz). This helped us identify the different sequences of period-luminosity relations, and allowed us to propose a calibration of the K magnitude with the observed large frequency separation. Conclusions. Interpreting period-luminosity relations in red giants in terms of solar-like oscillations allows us to investigate the time series obtained from ground-based microlensing surveys with a firm physical basis. This can be done with an analytical expression that describes the low-frequency oscillation spectra. The different behavior of oscillations at low frequency, with frequency separations scaling only approximately with the square root of the mean stellar density, can be used to precisely address the physics of the semi-regular variables. This will allow improved distance measurements and opens the way to extragalactic asteroseismology with the observations of M giants in the Magellanic Clouds. [ABSTRACT FROM AUTHOR]

Aims. In some scenarios for the formation of the Milky Way bulge, the stellar population at the edges of the boxy bulge may be younger than those on the minor axis or close to the Galactic center. So far the only bulge region where deep color-magnitude diagrams have been obtained is indeed along the minor axis. To overcome this limitation, we aim to age-date the bulge stellar populations far away from the bulge minor axis. Methods. Color-magnitude diagrams and luminosity functions have been obtained from deep near-IR VLT/HAWK-I images taken at the two Southern corners of the boxy bulge, i.e., near the opposite edges of the Galactic bar. The foreground disk contamination has been statistically removed using a pure disk field observed with the same instrument and located approximately at similar Galactic latitudes of the two bulge fields and ~30° in longitude away from the Galactic center. For each bulge field, mean reddening and distance are determined using the position of red clump stars, and the metallicity distribution is derived photometrically using the color distribution of stars in the upper red giant branch. Results. The resulting metallicity distribution function of both fields peaks around [Fe/H] ∼ -0.1 dex, with the bulk of the stellar population having a metallicity within the range: -1 dex ≲ [Fe/H] ≲ +0.4 dex, quite similar to that of other inner bulge fields. As for the previously explored inner fields, the color-magnitude diagrams of the two bar fields are consistent with their stellar population being older than ~10 Gyr, with no obvious evidence of younger population. Conclusions. The stellar population of the corners of the boxy bulge appears to be coeval with those within the innermost ~4° from the Galactic center. [ABSTRACT FROM AUTHOR]

We calibrated the absolute magnitudes M, M, $M_{K_{s}}$ and M of red clump stars in terms of colours. M and M are strongly dependent on colour, while the dependence of M and $M_{K_{s}}$ on colour is rather weak. The calibration of the absolute magnitudes M and $M_{K_{s}}$ is tested on 101 RC stars in the field SA 141. The Galactic model parameters estimated with this sample are in good agreement with earlier studies. [ABSTRACT FROM AUTHOR]

Aims. We investigate the large-scale metallicity distribution in the Galactic bulge using large spatial coverage to constrain the bulge formation scenario. Methods. We use the VISTA variables in the Via Lactea (VVV) survey data and 2MASS photometry, which cover 320 sqdeg of the Galactic bulge, to derive photometric metallicities by interpolating the (J - Ks)0 colors of individual red giant branch stars based on a set of globular cluster ridge lines. We then use this information to construct the first global metallicity map of the bulge with a resolution of 30' × 45'. Results. The metallicity map of the bulge revealed a clear vertical metallicity gradient of ~0.04 dex/deg (~0.28 dex/kpc), with metalrich stars ([Fe/H] ~ 0) dominating the inner bulge in regions closer to the Galactic plane (∣b∣ < 5). At larger scale heights, the mean metallicity of the bulge population becomes significantly more metal poor. Conclusions. This fits in the scenario of a boxy bulge originating from the vertical instability of the Galactic bar, formed early via secular evolution of a two-component stellar disk. Older metal-poor stars dominate at higher scale heights due to the non-mixed orbits of originally hotter thick disk stars. [ABSTRACT FROM AUTHOR]

Red clump (RC) giants are excellent standard candles in the Milky Way and the Large Magellanic Cloud. The near-infrared K-band intrinsic luminosity of RC giants exhibits only a small variance and a weak dependence on chemical composition and age. In addition, RCs are often easily recognizable in the color–magnitude diagrams of open clusters, which renders them extremely useful distance indicators for some intermediate-age or old open clusters. Here we determine the distance moduli of five Galactic open clusters covering a range of metallicities and ages, based on RC giants in the cluster regions using 2mass photometric data. We compare our result with those from main-sequence fitting and also briefly discuss the advantages and disadvantages of RC-based cluster distance determination. [ABSTRACT FROM PUBLISHER]

Red-clump (RC) giants are intermediate-age, core-helium-burning stars. The RC can be used as a standard candle. In particular, the small variance of the RC's K-band intrinsic luminosity and its weak dependence on chemical composition and age make it an extremely useful distance indicator. In this paper, we use 2mass data to search for RC stars in a sample of 60 Galactic open clusters with known reddening, ages, and distances, and obtain an average value for the RC's absolute Ks-band magnitude, MKs = −1.72 ± 0.17 mag. [ABSTRACT FROM PUBLISHER]